This invention relates to glass for use as a color cathode ray tubes, and in particular, to X-ray absorbing glass for use as panels of color television tubes having a selective light absorption and a controlled chromaticity value.
In a conventional color cathode ray tube, such as a color TV tube, a plurality of sets of three red-, green- and blue-phosphor materials are deposited on the inner surfaces of the face or panel glass to form a phosphor coating for emitting red, green and blue lights. When the phosphor coating is excited by electron beams from electron guns mounted within the glass tube, a colored image is produced and is visible through the panel glass.
Such a color cathode ray tube must be able to display a clear color picture image under a usual illumination such as a fluorescent lamp or even in daylight. Accordingly, a color cathode ray tube is desired to have a high brightness and a good contrast while suppressing a reflection of an external light. To that end, it is desired that a glass bulb of a color cathode ray tube should be highly transparent for the red color light (the wavelength of about 610-780 m.mu.), the green color light (about 500-570 m.mu.) and the blue color light (about 430-460 m.mu.) emitted from the phosphor materials, but should fully absorb the color light of the other wavelengths, for example, the yellow light (near 580 m.mu.) such as the sunlight at dusk, to which the human eyes are most sensitive as accompanied by dazzling, to clearly reproduce the red, green and blue colors.
In order to preserve the contrast from degrading by any ambient light, it has been known in the prior art to add NiO and CoO to a panel glass of a color cathode ray tube so as to color the panel glass to decrease its light transmission characteristic. The colored glass has a chromaticity of the neutral color. However, since it exhibits a uniform transmission characteristic for all of lights of the visible spectrum, the red, the blue, and the green lights emitted from the phosphors are also absorbed so that the brightness of the image produced may be disadvantageously degraded.
Another known solution to maintain the high contrast even under any ambient light is to add 3-5 wt% neodynium oxide (Nd.sub.2 O.sub.3) to a filter glass or a face glass of soda-lime-silica glass so that the glass may have selective absorption and transmittance properties by which the transmission of light in the red, the green, and the blue portions of the spectrum is permitted at a high rate but the transmission of the yellow portion is reduced, as disclosed in British Pat. No. 1,154,500. It is also disclosed in BRD Pat. No. 1,286,539 to add 2-8 wt% Nd.sub.2 O.sub.3 to the panel glass of a color TV tube.
In U.S. Pat. No. 3,143,683, it is disclosed to add 0.05-0.4 wt% Fe.sub.2 O.sub.3, 0.3-1.2 wt% Nd.sub.2 O.sub.3, 0.01-0.05 wt% NiO, 0.002-0.006 wt% CoO, and 0.002-0.02 wt% Se to a panel glass of lime-soda-silica glass in order to produce the desired transmission properties in the glass.
However, neither one of the glasses disclosed in British Pat. No. 1,154,500, BRD Pat. No. 1,286,539 and U.S. Pat. No. 3,143,683 has an X-ray absorption and a resistance to electron and/or X-ray browning.
A conventional color cathode ray tube had suffered from problems that X-rays leak from the tube and that its face glass browns or discolors during long use, because electron beams are emitted to the face glass in the tube. In order to resolve such problems, glass including at least one of SrO, BaO, PbO, ZnO, ZrO.sub.2 or WO.sub.3 has been proposed and practically used which exhibits a high absorption of X-ray and an excellent resistance to electron and/or X-ray browning, as disclosed in, for example, U.S. Pat. Nos. 3,464,932, 3,663,246, 3,723,354 and others.
While, in use of the glass including Nd.sub.2 O.sub.3 for the face glass of the color cathode ray tube as disclosed in British Pat. No. 1,154,500, BRD Pat. No. 1,286,539 and U.S. Pat. No. 3,143,683, the contrast and the color purity of the image produced by the three phosphor materials are not degraded by any ambient light illumination because of the selective light absorbing properties. However, the glass including Nd.sub.2 O.sub.3 does not exhibit the neutral color for tricolor phosphor P22 illuminant, that is, its chromaticity point on the x and y coordinates by ICI (International Commission on Illumination) is different from that of the P22 per se. This means that a desired color light is correctly formed by controlled emission of the three red-, green-, and blue-phosphors.
Furthermore, the glass including Nd.sub.2 O.sub.3 exhibits different colors by illumination of different neutral lights, for example, it is pinkish under the sunlight but bluish under the light from a fluorescent lamp. Such a nature of the glass as exhibits different colors under the sunlight and the other usual neutral illumination, will be referred to as "dichroism" hereinafter. Therefore, the color image produced by a color TV tube which uses the panel glass including Nd.sub.2 O.sub.3 varies in its color and contrast in dependence on whether it is viewed under the sunlight or a fluorescent lamp.
Therefore, even if the teaching in British Pat. No. 1,154,500 and others is merely applied on the X-ray absorbing and browning resistance glass as disclosed in U.S. Pat. No. 3,464,932 and others, no glass is obtained which exhibits the neutral color for P22 illuminant and which does not exhibit the dichroism for the sunlight and the light of a fluorescent lamp.